1. Field of the Invention
The invention relates to devices, techniques, and treatments for reducing or eliminating electromagnetic interference.
2. Relevant Background
Electromagnetic interference (EMI) can arise in any electronic system, either directly from circuitry or indirectly by conduction along connecting cables and by radiation. EMI can also arise from external sources to create problems in various systems. Undesirable signals occur whenever interference has a source, a receiver, and a transfer path. Accordingly, EMI can be reduced or eliminated by suppressing interference at the source, protecting the receiver against interference, and by reducing transmission.
Interference can propagate by radiation of electromagnetic waves in free space and by conduction on a conductive pathway. Techniques for suppressing radiated interference include shielding with conductive or absorbing materials such as conductive adhesive tapes, wire mesh, and gaskets. Techniques for suppressing conducted interference includes ferromagnetic cable shields, connector backshields, filtered connectors, ferrite toroids, and feedthrough capacitors that all reduce emissions conducted onto connecting cables. Conducted emissions generally concern signal frequencies of up to 30 MHz while radiated emissions have a frequency range of generally over 30 MHz.
Electromagnetic compatibility is regulated throughout the world. European Norms (EN) define regulations applicable in all European Union (EU) and European Free Trade Associated (EFTA) countries. Federal Communications Commission (FCC) regulates electromagnetic compatibility in the United States. Other agencies regulate emissions in other countries throughout the world.
Fundamentally, EMI should be addressed using good design practice to eliminate interference in design requirements. Design practice may be ineffective for interference that is directly related to inherent operating principles and for interference that is not detected until the final design phase. Further, combining components into systems not originally anticipated may cause unexpected EMI problems. Additional suppression may be needed using extra suppression components such as ferrites, capacitors, or shielding elements.
Referring to FIG. 9, a pictorial diagram illustrates several examples of conventional wire and cable shielding devices including toroids. Conventional techniques for improving electromagnetic noise emission involve placement of a ferrite toroid on the wire or cable. The toroid solution reduces EMI in some frequency ranges but has the disadvantages of increased material costs, increased assembly costs, and difficulty in installation from complexity in routing and dressing. Toroids are inconvenient since a cable that is intended to pass through a hole of only slightly larger diameter than the cable can no longer pass when the toroid is installed. A toroid is a ring-shaped solid lossy element that is commonly manufactured by combining a ferromagnetic powder into a substrate, and placing the combination into a mold for sintering at high pressure and temperature. One or more toroids can be placed around the cable, either by sliding the ring over the end of a cable or by snapping a split toroid into place on the cable. Toroids are typically ceramics that have the disadvantage of high fragility; the ferrite material commonly used for toroids can fracture in use. A fractured toroid has greatly reduced attenuation capability. For long cables, multiple toroids must be used to remedy differential-to-common mode conversion, increasing cost and complexity while reducing reliability.
Toroids can be porous, possibly resulting in moisture absorption or requiring shrink tubing overlying the toroids for physical and moisture protection. Other, similar applications of solid lossy materials have similar problems of cost, fragility, and installation difficulty.